The, a and an, are determiners, specifically, they are called articles. There are many determiners in English and they function somewhat like adjective in as much as they determine something about the noun they are modifying.
All determiners, including the two articles, refer to nouns in either a specific sense or a general sense.
'The' is specific. It is used to refer to specific things we know about. For example, if I say pass me the hammer," I'm talking a about a specific hammer, the one that both I and the person I'm speaking to know about. But if a say "pass me a hammer," I am not asking for a specific hammer, it could bey any hammer.
A and an, are non specific, or general. Let's say I have a three chocolate candies, one with a nut, one with a cherry and one is just pure chocolate. I ask you, "Would you like a chocolate?" It could be any one of the three. You look at the chocolates and ask, "can I have the cherry chocolate?" You use 'the' because you are referring to a specific chocolate.
A or an are used in the singular only. We do not say, 'can I have a chocolates?' We say, 'can I have a chocolate?' The can be used with both plural and singular nouns. "Can I have all of the chocolates?"
We do not use 'the' with proper nouns, titles or the names of places countries or things unless they include 'the' in the title or name. So, it is The United States but not The China. It's just China because that is the full name of the country.
Also we to not usually use articles with collective nouns, like chocolate, sunshine, water, pie, and so on. We don't say, 'I need the water,' or 'I need a water.' We just say, 'I need water.'